Late complications in the treatment of Hodgkin's disease with radiation therapy are increasingly being recognized. Thyroid dysfunction following irradiation is one such complication which has not been widely recognized, probably because of the subtle clinical nature of minor thyroid hormone imbalance. Two hundred thirty-two patients with Hodgkin's disease followed at BCRP have been evaluated for post-therapy thyroid dysfunction. Forty-one percent of those with mantle radiation therapy had an elevated TSH, but normal T4, but an additional 25% had an elevated TSH plus a depressed T4. Thus, 66% of 169 patients had evidence of thyroid dysfunction following mantle radiation therapy. One patient has been detected with irradiation-associated thyroid carcinoma. He had received initial irradiation in 1962.